HSQLDB is a relational database engine written in Java, with a JDBC driver, conforming to ANSI SQL:2008. A small, fast, multithreaded engine and server with memory and disk tables, LOBs, transaction isolation, multiversion concurrency and ACID.

phpPgAdmin is a fully functional web-based administration utility for a PostgreSQL database server. It handles all the basic functionality as well as some advanced features such as triggers, views, and functions (stored procs)

SQLTools is a light weight and robust frontend for Oracle database development. This program is not so comprehensive as Tora or TOAD but it is ideal when you need basic data querying capabilities and objects browsing. It also supports a basic subset of SQL*Plus commands (connect/disconnect, bind and define variables).
It works with Oracle 8i/9i/10g/11g databases. Oracle Client 32bit is required.
Check http://www.sqltools.net for more information,
http://www.sqltools.net/cgi-bin/yabb25/YaBB.pl?board=sql_status for development progress and news.

TOra is a Toolkit for Oracle which aims to help the DBA or developer of database applications. Features PL/SQL debugger, SQL worksheet with syntax highlighting, DB browser and a full set of DBA tools. TOra also includes support for MySQL and Postgres.

The best developer's or admin's GUI tool for Sqlite3 (sqlite) in the world. No joking here (or just a bit only) - it contains the most complete feature set of all tools available. Visit project homepage for more info, bugtracker, documentation, etc

SchemaSpy analyzes database metadata to reverse engineer dynamic Entity Relationship (ER) diagrams. It works with just about any JDBC-compliant database (Oracle/MySQL/DB2/SQL Server/PostgreSQL/Sybase/etc) and can identify Ruby on Rails relationships.

Racktables is a nifty and robust solution for datacenter and server room asset management. It helps document hardware assets, network addresses, space in racks, networks configuration and much much more!

aSQLiteManager - a SQLite manager for the Android platform. If the database is stored on the SDCard you can browse the data, change records and query the data. By installing aShell you can also edit system databases on rootet phones

MyWebSQL is a web based WYSIWYG Database client written in PHP. It has a simple and intuitive interface with the look and feel of a desktop application, and offers rich features for database management. No installation is needed, just download, extract and you are ready to go. It even has a single file compact version with full functionality, which you can quickly deploy on your server.
Currently MyWebSQL supports working with MySQL, PostgreSQL and SQLite databases.